Not as far as I am aware, there are no holes or blanking plates, it has been designed this way. I have found that if the unit is stood upright with the Netgear Logo pointing in the direction you want the strongest signal, it works really well.
I doubt it, but why not ask Netgear? In order to obtain a good signal for my tablet in my workshop (brick built, separate from house), I have used a small Netgear booster in a 13Amp plug half way between the router and the workshop. Total distance between router and tablet in workshop is about 18 metres. This works well. The router is located on first floor of the house and the tablet on ground floor. Signal has to go through 4 brick walls.
The antenna on my previous Netgear router could be unscrewed and replaced with a more powerful version. This does not appear to be possible with the N300 - I didn't want to force it in case it broke! If you use a desktop PC, you could try a different antenna on the wireless network card - or on a laptop, use an external USB wireless device.